Performing measurements on fluid samples is desirable in many oil industry applications. In the prior art such measurements are typically made by bringing samples to the surface using sealed containers, and sending the samples for laboratory measurements. A number of technical and practical imitations are associated with this approach.
The main concern usually is that the sample(s) taken to the surface may not be representative of the downhole geologic formation due to the fact that only limited sample material from a limited number of downhole locations can be extracted and taken to the surface. Thus, taking samples to the surface is impractical if it is desired to measure the fluid on a dense grid of sample points. Therefore, by necessity the measurements will only provide an incomplete picture of the downhole conditions.
In addition, these samples frequently contain highly flammable hydrocarbon mixtures under pressure. Depressurizing the containers frequently leads to the loss of the gas content. Handling of such test samples can be hazardous and costly.
It is therefore apparent that there is a need for direct downhole fluid testing that would overcome these and other problems associated with prior art solutions.
Various methods exist for performing downhole measurements of petrophysical parameters of the geologic formation. Nuclear magnetic resonance (NMR) logging is among the most important methods which have been developed for a rapid determination of such parameters, including formation porosity, composition of the formation fluid, the quantity of movable fluid, permeability and others. At least in part this is due to the fact that NMR measurements are environmentally safe and are unaffected by variations in the matrix mineralogy. In a typical NMR experiment a logging tool is lowered into a drilled borehole to measure properties of the geologic formation near the tool. The tool is pulled up at a known rate and measurements are continuously taken and recorded in a computer memory, so that at the end of the experiment a complete log is generated which shows the properties of the geologic formation along the length of the borehole. Alternatively, NMR logging can be done while the borehole is being drilled.
NMR logging is based on the observation that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter T.sub.1, known as the spin-lattice relaxation time. Another related and frequently used NMR logging parameter is the spin-spin relaxation time constant T.sub.2 (also known as transverse relaxation time) which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool. Both relaxation times provide indirect information about the formation porosity, the composition and quantity of the formation fluid, and others.
Another measurement parameter used in NMR well logging is the formation diffusion. Generally, diffusion refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. Self-diffusion of a fluid is directly related to the viscosity of the fluid, a parameter of considerable importance in borehole surveys. In a uniform magnetic field, diffusion has little effect on the decay rate of the measured NMR echoes. In a gradient magnetic field, however, diffusion causes atoms to move from their original positions to new ones, which also causes these atoms to acquire different phase shifts compared to atoms that did not move. This contributes to a faster rate of relaxation.
It has been observed that the mechanisms which determine the values of T.sub.1, T.sub.2 and diffusivity depend on the molecular dynamics of the sample being tested. In bulk volume liquids, typically found in large pores of the formation, molecular dynamics is a function of molecular size and inter-molecular interactions which are different for each fluid. Thus, water, gas and different types of oil each have different T.sub.1, T.sub.2 and diffusivity values. On the other hand, molecular dynamics in a heterogeneous media, such as a porous solid which contains liquid in its pores, differs significantly from the dynamics of the bulk liquid and generally depends on the mechanism of interaction between the liquid and the pores of the solid media. It will thus be appreciated that a correct interpretation of the measurement parameters T.sub.1, T.sub.2 and diffusivity can provide valuable information relating to the types of fluids involved, the structure of the formation and other well logging parameters of interest.
NMR measurements of geologic formations can be done using, for example, the centralized MRIL.RTM. tool made by NUMAR, a Halliburton company, and the sidewall CMR tool made by Schlumberger. The MRIL.RTM. tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications including: "Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination," by Miller, Paltiel, Millen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sept. 23-26, 1990; "Improved Log Quality With a Dual-Frequency Pulsed NMR Tool," by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sep. 25-28, 1994). Details of the structure and the use of the MRIL.RTM. tool are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115 and 5,557,200 all of which are commonly owned by the assignee of the present invention. The Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 5,055,787 and 5,055,788 to Kleinberg et al. and further in "Novel NMR Apparatus for Investigating an External Sample," by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992. The content of the above patents are hereby expressly incorporated by reference.
Wireline logging of boreholes performed using the NMR tools described above or other techniques known in the art provides valuable information concerning the petrophysical properties of the formation and in particular regarding the fluid composition of the formation. Additional fluid parameter information can be critical for the interpretation of the wireline NMR measurements. For example, it is often desirable to distinguish between water, connate oil, drilling mud filtrates and gas based on the differences in T.sub.1, T.sub.2 and diffusivity. The true values for connate oil and the drilling mud filtrates under reservoir conditions are often unknown and must be approximated from laboratory measurements done under different conditions. Therefore, for increased accuracy, it is desirable to perform real-time downhole NMR determination of the T.sub.1, T.sub.2 and diffusivity parameters of borehole fluids to enhance the quality and reliability of the formation evaluation obtained using the standard measurements.
Direct downhole measurements of certain fluid properties is known in the art. Several commercially available tools can be used to this end. Examples include the RDT tool manufactured by Halliburton, the Reservoir Characterization Instrument (RCI) from Western Atlas, and the Modular Formation Dynamics Tester (MDT) made by Schlumberger. These tester tools have modular design that allows them to be reconfigured at the well site. Typically, these tools provide pressure-volume measurements, which can be used to differentiate liquids from gases, and are also capable of providing temperature, resistivity and other mechanical or electrical measurements. However, none of these tools is presently capable of providing NMR measurements, such as hydrogen density, self diffusivity or relaxation times.
Therefore, there is a need for a tester capable of performing direct downhole NMR measurements that can be used to enhance the quality and reliability of formation evaluation obtained using prior art techniques. Additionally, there is a need to provide a modular NMR downhole tester that can be used as an add-on to existing testing equipment so as to minimize the cost of the extra measurements.